The present invention relates to ferroelectric materials and methods for fabrication thereof, and more specifically to ferroelectric materials with a chemical formula:
A(1xe2x88x92x)BxC(1xe2x88x92y)DyF3;
such as NaCF3, or Na1xe2x88x92xKxCaF3, or which are fabricated by deposition of source materials onto a substrate with a (111) orientation and a cubic lattice constant of between about 3.8 to 4.3 Angstroms, or a substrate with a (001) orientation and a hexagonal lattice with an a-axis parameter of between about 5.4 to 6.2 Angstroms.
It is first to be appreciated that ferroelectric materials are noted for having large permittivities and for demonstrating hysteresis-type retention of residual polarization after an electric field which has been applied, is removed. These properties make ferroelectric materials attractive for application in, for instance, thin film capacitors and random access memories (RAM""s), infrared sensors based upon pyroelectric properties, and possibly thermal infra-red switches based on electro-optic properties, as well as microactuators based upon piezoelectric properties.
All ferroelectric materials are related by having noncentrosymmetric crystalline structure and attendant polarization due to displacement of metal cations from the center of their respective coordination spheres. Many ferroelectric materials with formulae ABX3 occur in distorted perovskite structures composed of octahedra connected through shared vertices. Typically small, highly charged, metal cations, (such as Ti4+) are located within the octahedra and large cations, (such as Ba2+), are located in the spheres between octahedra. Polarization occurs when the small cation is displaced toward one corner or face of the octahedral environment. BaTiO3 is the archetypical ferroelectric material of this variety.
Because ferroelectric properties are related to noncentrosymmetric crystalline structure, phase transitions to higher symmetry centrosymmetric structures typically destroy their ferroelectric properties. Consequently, most ferroelectric materials have a phase transition Curie temperature (Tc), above which the materials no longer act as a ferroelectric, but might retain good dielectric properties.
It is also noted that most ferroelectric materials are oxides and are opaque in ultraviolet spectral region. A ferroelectric material which is transparent in the ultraviolet spectral region could therefore provide additional utility, and, as taught by the present invention, candidates for such a ferroelectric material include fluorides.
Continuing, as early as 1984 Dr. J. W. Flocken, Dr. R. A. Guenther at the University of Nebraska at Omaha, and Dr. L. L. Boyer of the Naval Research Lab determined that of approximately sixty (60) halide-based perovskites of the form ABX3, where A is an alkali metal, and B is an alkaline earth metal, and X is a halogen, three (3) were candidates for study as ferroelectrics. One of these compounds is NaCaF3. This is because NaCaF3 was determined to be of a distorted perovskite structure, the distortion being from a cubic symmetry to a rhombohedral symmetry by elongation along the cubic body-centered diagonal. The body-diagonal of the cubic cell being the trigonal axis of the ferroelectric structure, along which polarization is expected to occur by displacement of sodium cations and fluoride anions in opposite directions from the cubic geometry.
In view of the potential benefits associated with NaCaF3, several attempts to fabricate NaCaF3 over a period of years were made, however, none produced results. Unsuccessful approaches included solid-state co-precipitation, freeze-drying, and decomposition reactions.
With the present invention in mind, a Search of Patents was performed, with the result being that very little was found. Perhaps the best reference is U.S. Pat. No. 3,238,015 to Pessahovitz et al., which describes the preparation of KFMgF2 by dissolving thixotropic MgF2 in a hot solution of KF. The resultant material, however, is not identified as being ferroelectric. Another U.S. Pat. No. 3,682,727 to Phillips describes fabrication of photochromic materials by melting CaF2, LaF2 and NaF in a crucible at between 630 and 725 degrees Centigrade. U.S. Pat. No. 5,888,296 to Ooms et al. describes formation of a layer of ferroelectric bismuth on a lattice matched semiconductor. U.S. Pat. Nos. 5,667,725 and 5,552,083 to Wanatabe et al. describe the preparation of colloidal NaFMgF2. A U.S. Pat. No. 5,356,831, to Calveillo et al., describes lattice matching a semiconductor to a substrate such as MgO with a lattice constant in the range of three (3) to six (6) Angstroms. Recent Patents which describe application of ferroelectric materials in thin film capacitors and RAM devices are U.S. Pat. Nos. 5,889,299 and 5,889,696.
In addition, a number of Scientific Papers have been identified as follows:
xe2x80x9cThe Physics Of Ferroelectric Memoriesxe2x80x9d, by Auciello, Scott and Ramamoorthy, Physics Today, (July 1998).
xe2x80x9cTheory For Forces Between Closed-Shell Atoms And Moleculesxe2x80x9d, Gordon and Kim, J. Chem. Phys., Vol. 56, No. 6, (March 1972).
xe2x80x9cPhase Transitions In Mixed Alkali Calcium Trifluoride Solid Solutionsxe2x80x9d, Flocken, Smith, Hardy, Stevenson and Swearingen, Mat. Research Bull., Vol 31, No. 9, (1996).
xe2x80x9cMolecular Dynamics Simulation Of Superionicity In Neighboring NaMgF3xe2x80x9d, Zhou, Hardy and Cao, Geophysical Research Lett., Vol. 24, No. 7, (April 1997).
xe2x80x9cSynthesis Of Novel Thin-Film Materials By Pulsed Laser Depositionxe2x80x9d, Lowndes, Geohegan, Puretzky, Norton and Rouleau, Science, Vol 273 (August 1996).
xe2x80x9cThin Film Synthesis Of Metastable And Artificially Structured Oxidesxe2x80x9d, Gupta, Elect. Mat., Curr. Opin. Solid State Mater Sci., Vol 2 (1997).
xe2x80x9cStabilization of YMnO3 In A Perovskite Structure As A Thin Filmxe2x80x9d, Salvador, Doan, Mercey and Raveau, Chem. Mater., Vol. 10, No. 10, (1998).
xe2x80x9cPulsed Laser Ablation Synthesis Of NbNx (0 (x (1.3) Thin Filmsxe2x80x9d, Chem. Mater., Vol. 6, No. 12, (1994).
xe2x80x9cNew Phase Superconducting NbN Stabilized By Heteroeptaxial Film Growthxe2x80x9d, Phys. Rev. B, Vol. 51, No. 14, (April 1995).
xe2x80x9cPulsed Laser Deposition Of Oriented PbZr54Ti46O3xe2x80x9d, Grabowski, Horwitz, and Chrisey, Ferroelectrics, Vol. 116, (1991).
xe2x80x9cMicrowave Properties Of Sr0.5Ba0.5TiO3 Thin-Film Interdigitated Capacitorsxe2x80x9d, Kirchoefer, Pond, Carter, Chang, Agrwal, Horowitz and Chrisey, Microwave and Opt. Tech. Lett., Vol. 18, No. 3 (June 1998).
xe2x80x9cEpitaxial Growth Of Metal Fluoride Thin Films By Pulsed-Laser Depositionxe2x80x9d, Norton, Budai, Chakoumakos, Geohegan and Puretzky, Mat. Res. Soc. Symp. Proc. Vol. 387, (1996).
xe2x80x9cPredicting New Materialsxe2x80x9d, Boyer, Computers in Phys. Vol 8, No. 1, (January/February 1994).
xe2x80x9cFirst-Principals Study Of Structural Instabilities In Halide-Based Perovskites: Competition Between Ferroelectricity and Ferroelasticityxe2x80x9d, Guenther, Hardy, Boyer, Phys. Rev. B, Vol. 31, No. 11, (June 1985).
xe2x80x9cFerroelectricity In Perovskites like NaCaF3 Predicted Ab Initioxe2x80x9d, Phys. Rev. B, Vol 39, No. 13, (May 1989).
xe2x80x9cFerroelectric Phase Transitions In NaCa-Halide Perovskitesxe2x80x9d, Flocken, Guenther, Hardy, Edwardson and Boyer, Phase Transactions, Vol. 20, (1990).
xe2x80x9cFerroelectric Phase Transactions In NA-CA-Halide Perovskitesxe2x80x9d, Flocken, Mei, Guenther, Hardy, Edwardson and Boyer, Ferroelectrics, Vol. 104, (1990).
xe2x80x9cPerovskite To Antiperovskite In ABF3Compoundsxe2x80x9d, Boyer and Edwardson, Ferroelectrics, Vol. 104, (1990).
xe2x80x9cThe Effect Of K Defect Clusters On The Ferroelectric Phase Transition In NaCaF3xe2x80x9d, Flocken, Mei, Guenther, Hardy and Boyer, Ferroelectrics, Vol. 120, (1991).
xe2x80x9cRevised Effective Ionic Tadii and Systematic Studies of Interatomic Distances in Halides and Chalcogenidesxe2x80x9d, Acta Cryst A 32 (1976).
xe2x80x9cNew Phase of Superconducting NbN Stabilized by Hetroepitaxial Film Growthxe2x80x9d, Treece et al., Phys. Rev. B, Vol 51, No. 14 (April 1995).
xe2x80x9cPerovskite to Antiperovskite in ABF3 Compoundsxe2x80x9d, Boyer et al., Ferroelectrics, Vol. 104, (1990).
xe2x80x9cThe Crossover of Phase Transitions From NaCaF3 to KCaF3xe2x80x9d, Flocken et al., Ferroelectrics, Vol. 120 (1991).
Even in view of the literature, a need remains for a method by which ferroelectric materials with a chemical formula: A(1xe2x88x92x)BxC1xe2x88x92y)DyF3; such as NaCaF3 or Na1xe2x88x92xKxCaF3 can be fabricated.
The present invention is primarily ferroelectric NaCaF3, which has a Crystalline Structure similar to LiNbO3, (eg. perovskite with distortion away from cubic toward rhombohedral), produced by deposition of NaF and CaF2 onto a lattice matched substrate, wherein use of a lattice matching substrate is essential to overcome adverse thermodynamic forces.
To the Inventor""s knowledge no-one has ever before fabricated even thin-film NaCaF3, although theory has existed for some time which predicts that, were its fabrication found to be possible, the result would be Ferroelectric. Recently, however, Inventor, Dr. Robert Smith, a Naval Reservist and Researcher at the University of Nebraska at Omaha, utilizing Pulsed Laser Deposition (PLD) techniques at the Naval Research Lab (NRL) in Washington D.C. under an Army Research Grant, successfully fabricated thin films of NaCaF3 on a (111) orientation MgO substrate. Said (111) orientation MgO substrate was suggested by Dr. J. S. Horwitz, a civilian employee at the NRL. It is noted that other deposition techniques should also work, and that (111) orientation SrZrO3 substrate would probably be preferable as a substrate, based upon cubic lattice dimensions. (111) orientation SrZrO3, however, is not readily available in a form adaptable for use as a substrate. Other possible substrate candidates include (111) orientation SrTiO3 which has an a-axis parameter of 3.905, (111) orientation Au, and generally any (111) orientation substrate with a cubic lattice constant of between about 3.8 to 4.3 Angstroms, or any (001) orientation substrate with a hexagonal a-axis parameter of between 5.4 and 6.2 Angstroms.
In work performed to date, said (111) orientation MgO substrate was utilized because its cubic lattice constant acts as a thermodynamic stability providing template upon which thin film NaCaF3 can be achieved. Again however, in general, any substrate with a (111) orientation and cubic lattice constant of between about 3.8 to 4.3 Angstroms, (nominally about 4.05), or any substrate with a (001) orientation hexagonal lattice with an a-axis parameter of between about 5.4 to 6.2, (nominally about 5.8) is applicable. It is noted that bulk NaCaF3 has yet to be fabricated, and the difficulty in doing so is most likely because of thermodynamic instabilities associated with tertiary fluorides.
The present invention is generally a ferroelectric material having a formula A(1xe2x88x92x)BxC1xe2x88x92y)DyF3; where A and B are each independently selected from Periodic Chart Group IA, and said C and D are each independently selected from Periodic Chart Group IIA, said ferroelectric material being in functional combination with a substrate with a substantially lattice matching substrate.
As specific examples said present invention ferroelectric material can provide that:
A is Sodium (Na);
x=0.0;
C is Calcium (Ca); and
y=0.0;
and said substrate is selected from the group consisting of:
(111) orientation MgO;
(111) orientation SrZrO3;
(111) orientation SrTiO3;
(111) orientation Au;
a (111) orientation substrate with a cubic lattice constant between about 3.8 and 4.3 Angstroms; and
a (001) orientation substrate with a hexagonal a-axis parameter of between about 5.4 and 6.2 Angstroms.
As further specific examples said present invention ferroelectric material can provide that:
A is Sodium (Na);
B is Potassium (K);
x is between 0.0 and 1.0;
C is Calcium (Ca); and
D is selected from the group consisting of a member of the Periodic Chart Group IIA; and
being absent; and
y is between 0.0 and 1.0;
and said substrate is selected from the group consisting of:
(111) orientation MgO;
(111) orientation SrZrO3;
(111) orientation SrTiO3;
(111) orientation Au;
a (111) orientation substrate with a cubic lattice constant between about 3.8 and 4.3 Angstroms; and
a (001) orientation substrate with a hexagonal a-axis parameter of between about 5.4 and 6.2 Angstroms.
A present invention method of fabricating a ferroelectric material having a formula
A(1xe2x88x92x)BxC(1xe2x88x92y)DyF3;
where A and B are each characterized by a selection from the group consisting of:
each being independently selected from Periodic Chart Group IA;
one being selected from Periodic Chart Group IA and the other being absent;
and said C and D are each characterized by a selection from the group consisting of:
each being independently selected from Periodic Chart Group IIA;
one being selected from Periodic Chart Group IIA and the other being absent;
said ferroelectric material being in functional combination with a substrate selected from the group consisting of:
(111) orientation Au;
a (111) orientation substrate with a cubic lattice constant between about 3.8 and 4.3 Angstroms; and
a (001) orientation substrate with a hexagonal a-axis parameter of between about 5.4 and 6.2 Angstroms;
said method comprising the steps of:
a. providing a pulsed laser deposition system;
b. placing target containing A and C, and optionally B and/or D into said pulsed laser deposition system;
c. placing a selected substrate into said pulsed laser deposition system;
d. causing pulses of laser energy to ablate said target containing A and C, and optionally B and/or D to the end that ablated A and C, and optionally B and/or D deposit onto said substrate.
The present invention further includes a ferroelectric material having a formula
A(1xe2x88x92x)BxC(1xe2x88x92y)DyF3;
where A and B are each characterized by a selection from the group consisting of:
each being independently selected from Periodic Chart Group IA;
one being selected from Periodic Chart Group IA and the other being absent;
and said C and D are each characterized by a selection from the group consisting of:
each being independently selected from Periodic Chart Group IIA;
one being selected from Periodic Chart Group IIA and the other being absent;
said ferroelectric material being in functional combination with a substrate selected from the group consisting of:
(111) orientation Au;
a (111) orientation substrate with a cubic lattice constant between about 3.8 and 4.3 Angstroms; and
a (001) orientation substrate with a hexagonal a-axis parameter of between about 5.4 and 6.2 Angstroms;
which is fabricated by the method recited infra herein.
A preferred present invention ferroelectric material has a formula NaCaF3 and is a thin film in functional combination with a surface of a substrate, said substrate being selected from the group consisting of:
(111) orientation MgO;
(111) orientation SrZrO3;
(111) orientation SrTiO3;
(111) orientation Au;
a (111) orientation substrate with a cubic lattice constant between about 3.8 and 4.3 Angstroms; and
a (001) orientation substrate with a hexagonal a-axis parameter of between about 5.4 and 6.2 Angstroms.
Another present invention material, which so far has not demonstrated ferroelectric properties, has a formula comprising KCaF3 and is a thin film in functional combination with a surface of a substrate, said substrate being selected from the group consisting of:
(111) orientation MgO;
(111) orientation SrZrO3;
(111) orientation SrTiO3;
(111) orientation Au;
a (111) orientation substrate with a cubic lattice constant between about 3.8 and 4.3 Angstroms; and
a (001) orientation substrate with a hexagonal a-axis parameter of between about 5.4 and 6.2 Angstroms.
Additionally, a present invention ferroelectric material has a formula Na(1xe2x88x92x)KxCaF3 and is a thin film in functional combination with a surface of a substrate, said substrate being selected from the group consisting of:
(111) orientation MgO;
(111) orientation SrZrO3;
(111) orientation SrTiO3;
(111) orientation Au;
a (111) orientation substrate with a cubic lattice constant between about 3.8 and 4.3 Angstroms; and
a (001) orientation substrate with a hexagonal a-axis parameter of between about 5.4 and 6.2 Angstroms.
The present invention also includes a ferroelectric material having a formula
A(1xe2x88x92x)BxC(1xe2x88x92y)DyF3;
where A and B are each characterized by a selection from the group consisting of:
each being independently selected from Periodic Chart Group IA;
one being selected from Periodic Chart Group IA and the other being absent;
and said C and D are each characterized by a selection from the group consisting of:
each being independently selected from Periodic Chart Group IIA;
one being selected from Periodic Chart Group IIA and the other being absent;
which is characterized by X-ray diffraction fingerprint results corresponding to the presence of material(s) inherent in
A(1xe2x88x92x)BxC(1xe2x88x92y)DyF3;
The present invention will be better understood by reference to the Detailed Description Section of this Disclosure, in conjunction with the Drawings.
It is therefore a primary object and/or purpose of the present invention to teach ferroelectric materials with a chemical formula:
A(1xe2x88x92x)BxC(1xe2x88x92y)DyF3;
such as NaCaF3, or Na1xe2x88x92xKxCaF3, which are fabricated by, for instance, deposition of source materials onto a substrate with a (111) orientation and a cubic lattice constant of between about 3.8 to 4.3 Angstroms, (eg. Au), or a substrate with a (001) orientation and a hexagonal lattice with an a-axis parameter of between about 5.4 to 6.2 Angstroms.
It is another object and/or purpose of the present invention to teach a ferroelectric material, such as NaCaF3, which is formed by a pulsed laser technique.